What Cam for a 850 Mk2a ?

Hi All,
I'd like some advice and views on cam type for use in my 850 Mk11a. I know this issue has been dicussed many times but I would realy like to come up with a definative answer for my machine as I'm just about to order parts. Touring use with torque and reliability are my key aims here. The bike is used for distance work, solo and two up with lots of luggage. It has weekend blasts around Derbyshire, The Lakes, Yorkshire Dales and Scottish boarders with odd trips into Europe. I don't need to be racing ( got my Weslake Triton for that ! ) but just want a grunty general purpose motor. I currently run standard pistons and valves with a single Mk2 34mm Amal.
I am starting an engine and gearbox overhaul on my bike and really want to get the correct cam for the use I put the machine to. Some people I have spoken to say stick with a standard 850 commando cam while others have said while doing a rebuild it would be a missed opportunity not to fit a PW3 cam, others have said a well set up 2S will meet my needs. Reading through past posts it would appear that the PW3 needs higher compression ratios to gain any benefit whilst others have said it greatly improves torque on a standard motor. Help I'm confussed :roll:
At the moment I'm swaying towards the standard cam set up but any help or advice would be greatly appreciated before I spend my cash. Please remember I will be using the bike for 70 / 80 mph cruising, lots of luggage, A-roads folowed by twisty hill climbs and the odd hour on the motorway, it does the occasional 400miles in a day but were not racing Jap bikes anymore :shock:
Thanks in advance, Shaggy

with a single carb and stock compression I would stay with a stock cam. the other reason is the way you stated you was going to use the bike. a cam with more duration and over lap ( pw-3 ) will move the power higher up in the RPM range and suffer in the lower range. I also DON'T like to put in a hotter cam with out other mods to the engine and the FIRST is more compression with more cam. if you want more low end torque ad just a little more compression with every thing you already have. here in the states a nice low end torque motor would be an 850 or even 880 with 9 to 1 compression, a stock cam and 34 MM single carb

Windy exactly reflects my understanding of well set up 850 for long lasting normal torquy enjoyment. Bumping up cam profile requires upping CR or they fall flat to idle well or respond as nice to small throttle in low to mid range. About only concern would be proneness to detonation if upping CR much and not the cam too.
Combats with standard cam tend to detonate @ 10 CR, so don't go that high as bigger bore tend to detonate more than smaller.

Torque cams close the intake early to start compression early for more lower rpm bang. Aggressive cams need CR and some rpm to begin to pack in mixture with a late closing intake. I'd stick with small ports and maybe even a 32 mm single carb.
The smaller passages do not restrict flow but assist it until getting near red line limits of valve and crank but a joy until the power band falls off protectively.

I like Megacycle cams, Norris grind, flat followers, only a slight increase in duration and lift, stock springs, hard faced for reliability. Rick

I would stick with the stock cam. I had a hotter Megacycle put in on my last rebuild and I've regretted ever since. Lots of power at 4500 and above, but none of the nice torque that the stock cam gives.

Another nice push in seat of pants is a fast rise ignition curve like points example.
TriSpark seems about closest in electronics. I've been told a notched/sliced spray tube acts like a faux accelerator pump in 750's, but 850 already have them.
A real accelerator pump carb would likely knock ya socks off on low to mid range response. Not exactly a mileage feature if enjoying it a lot, but can't take the saved gasoline with ya. A bit tall gearing is very pleasant to me if torque on tap is eager.

shaggy, based on what you say you want from your Commando, I would stick with the standard cam. You don't say that you are trying to get more power from your Commando, so based on my very limited but slowly growing knowledge about these things, putting a different (performance) cam in the engine in isolation, is unlikeley to benefit performance anyway, and with the exception of increasing capacity, you should get more torque with the standard cam, and this would seem to suit the way you ride it?

The advantage with the standard cam is that you just fit it and don't have to check valve head to piston clearances, or grind out the cam shaft tunnel so that the lobes will clear, and although the valve train seems to tolerate the extra lift OK, there will not be that concern either. You could always speak to Norman White about the PW3, but I personally think there is more to do to get a noticable improved engine response than just fit the cam (such as increase C:R, and put twin carbs on in your case etc).

Caveat emptor, my opinion may be wrong! Don't ask me how I think I know

I like the 560-NR cam in the 850 even with the single carb, it has only 20 thou more lift and 12 degrees more duration than the stock cam, a small increase in power but a large increase in reliability. I've seen too many soft English made cams to trust them anymore.

If you want reliability then you you can do better than a stock cam. The cast iron usually doesn't last long and some of them are not heat treated correctly and fail. Some last and some don't.

A cam for radiused lifters will last much longer. I'm not talking 3" radius but 1-1/8" radius. The only cam I know of that has a performance curve close to stock specs is the "stock performance cam" for 1-1/8" radiused lifters.

See the specs here:
http://users.gotsky.com/jimschmidt/jsmo ... ifters.asp

Its very close to the stock Norton cam with a few more thousands of an inch lift and a couple of degrees more duration. Not enough to make it hotter (just healthy) and the low end grunt is still there. The different is that there is less stress on the valve train because of the lighter lifters. The radiused lifters make life easier on the cam (no pointy cam nose) and the cam is hard face welded (see photo and note the smooth rounded nose). You have to get the new lifters, tappet blocks and shorter pushrods but in the long run you save by not having to replace expensive worn out cams. If reliability is what you are after then this is it.



Jim

All I know about my bone-stock '75 MkIII 850 Interstate is that it hauls on down the road at 80 all day long (at 40+ MPG), and all you have to do is THINK ABOUT gassing it to climb fairly steep grades or pass 18-wheelers.

AllI know is a lot stock cams were worn through what little hardening they had in less than 15000 miles. It should not be up to the purchaser to check and correct hardening after buying a cam.

I wonder if stock cams could be nitrided then polished for more endurance w/o much extra cost. Flat tappets are better for torque configuration as they get the valves off seats faster than radius lifters. In lower rpm power making, lift don't matter so much as how soon the flow can get going. In lower rpms use valve train mass is not nearly the trouble and wear as hi rpm maxed out power seeking.
Geoff Collins of Offset Crankshaft does cams and nitriding.

In my old old age I'll be still craving slow speed grunt but not so much illegal rates.
Hi torque configurations are generally great to start and idle too.

When I changed to a stock performance cam with radiused lifters there was no loss of low end torque. You can grind a radiused cam so that it exactly matches the lift and duration of a flat lifter cam. Or you can grind a radiused lifter cam so that is has MORE low end torque than a peakier flat lifter cam. You can do anything you want with the profiles up to the paint where you run off the edge of the lifter. Running off the edge of the lifter only happens with High duration cams with all top end and no low end.

You should be able to put as much off the seat lift acceleration as you want in a radiused cam - more than you can with a heavier flat lifter - up to the point where things break. Its the stress that limits your lift acceleration and flat lifters create more stress than radiused lifters because of the extra weight - so flat lifter cams are limited by stress/accelleration problems more so than radiused lifter cams. Leo Goff made his own drag race cams that had so much lift acceleration that they could not be used in road racing.

Alrighty Jim, with special attention and cost its good to know radius lifters can do it all. You know I put my money where my mouth is - so I had some restless times looking at cost/profile benefit/availability for Ms Peel's weird needs, so I'm rationalizing to use flat lifters first time around.

Just wanted to say thanks to everyone for your input on this topic, it's been a great help.

Cheers Shaggy

For the use you are putting the bike to, I would stick with the stock cam, the PW, 2S (Combat cam), 3S and 4S cams all offer more performance, but at the expense of low end torque. You have probably noticed that a lot of people like to brag about what they have in their engine, Most don't ride more than 1-2000 miles a year! Make sure the cam you buy is from a reputable source and that the cam followers are in good shape. Use stock style followers. Small imperfections and wear marks on followers can be carefully stoned out. Assemble with smear of some high viscosity oil or, if available, STP upon assembly.